10. The Intestines

Question 1

What two routes can electrolytes, water and nutrients take from the gut lumen into the blood?

Answer 1

Cellular and paracellular.

Question 2

What 3 things result in the large surface area of the intestines to aid absorption?

Answer 2

Mucosa folded into villi, surface is covered with microvilli (brush boarder) and intestine has plicae circulares.

Question 3

In what form can carbohydrates be absorbed?

Answer 3

As monosaccharides.

Question 4

Where does the final breakdown of carbohydrates into monosaccharides occur and by what enzymes?

Answer 4

In the brush boarder by brush boarder enzymes.

Question 5

What ion is required for glucose to enter the brush boarder of the intestines?

Answer 5

Na+.

Question 6

Name 2 monosaccharides

Answer 6

Fructose.
Galactose.
Glucose.

Question 7

Name 2 common dietary carbohydrates

Answer 7

Starch (polysaccharide).
Lactose (disaccharide).
Sucrose (disaccharide).

Question 8

What are the two components of starch, and what type of bonds are found between each?

Answer 8

Amylose - alpha-1-4 bonds.
Amylopectin - alpha-1-4 bonds between horizontal chains, and alpha-1-6 bonds vertically between every two glucose (one maltose) molecules.

Question 9

What bonds does amylase break in starch digestion?

Answer 9

Alpha-1-4 bonds between glucose molecules in amylose.

Question 10

What is a common breakdown product of starch?

Answer 10

Maltose (two glucose molecules).

Question 11

What bonds does alpha dextrin break in starch digestion?

Answer 11

Alpha-1-4 bonds in amylopectin.

Question 12

What bonds does isomaltase break in starch digestion?

Answer 12

Alpha-1-6 bonds in amylopectin.

Question 13

With what brush boarder enzymes are the following products from initial starch digestion broken down into glucose:
Maltose?
Alpha dextrin?

Answer 13

Maltose - maltase.

Alpha dextrins - isomaltase.

Question 14

With what brush boarder enzymes are lactose and sucrose broken down, and into what?

Answer 14

Lactose - with lactase into glucose and galactose.

Sucrose - with sucrose into glucose and fructose.

Question 15

How is glucose absorbed by an enterocyte?

Answer 15

The Na+/K+ ATPase on the basolateral membrane maintains the low intracellular [Na+] by moving Na+ into the capillaries.
SGLT-1 binds Na+, which then allows glucose binding, and Na+ and glucose moves into the cell.
GLUT2 transports glucose out of the enterocyte, as glucose diffuses down its concentration gradient into the capillary blood.

Question 16

Fructose uses a different transporter to glucose to enter into the enterocyte. What transporter does it use?

Answer 16

GLUT5.

Question 17

How do oral rehydration fluids eg lucozade work?

Answer 17

Uptake of Na+ generates an osmotic gradient, and water follows. So the mixture of glucose and salt in the oral rehydration fluid stimulates maximum water uptake.

Question 18

In what form is protein absorbed into the blood?

Answer 18

As amino acids and small peptides.

Question 19

How are amino acids transported into enterocytes?

Answer 19

By Na+-amino acid co-transporters. There are different co-transporters for neutral, acidic, basic and imino amino acids.

Question 20

Most protein products are ingested as dipeptides and tripeptides, not as amino acids. How are dipeptides and tripeptides moved into enterocytes to be converted to amino acids?

Answer 20

Moved by H+ co-transporter - peptide transported 1.

Question 21

By what enzyme are dipeptides and tripeptides converted into amino acids inside enterocytes?

Answer 21

Cytosolic peptidases.

Question 22

How is water taken up?

Answer 22

Na+ is moved by active transport out of the enterocyte on the basolateral membrane, and so Na+ diffuses into the enterocytes, taking water with them. Water can then move into the intercellular space and blood. Fluid absorbed is isosmotic.

Question 23

Both the small and large intestines have Na+/K+ ATPase on the basolateral membrane of the enterocytes. However they transport Na+ differently on their apical membranes. How?

Answer 23

Small intestine - Na+ is co-transported.

Large intestine - Na+ channels, which are induced by aldosterone (more water absorbed to increase BP).

Question 24

How is calcium absorbed if calcium intake is normal or high?

Answer 24

By passive paracellular absorption.

Question 25

How is calcium absorbed if calcium intake is low?

Answer 25

By active transcellular absorption. Calcium enters the enterocyte vi facilitated diffusion, and Ca+ ATPase removed the Ca+ from the basolateral membrane, transporting it into the blood.

Question 26

Why is there a link between hypocalcaemia and vitamin D deficiency?

Answer 26

Process of absorbing calcium when calcium intake is low requires vitamin D, which is transported from the blood into the enterocyte. This is stimulated by parathyroid hormone. Therefore vitamin D deficiency can lead to hypocalcaemia if calcium intake is also low.

Question 27

How is iron absorbed by enterocytes?

Answer 27

Absorbed across the apical membrane co-transported with H+.

Question 28

What happens to the iron absorbed by enterocytes when iron levels are low?

Answer 28

Iron binds to transferrin and is transported to be stored in ferritin complexes in the bone marrow, liver and spleen.

Question 29

What happens to the iron absorbed by enterocytes when iron levels are high?

Answer 29

Iron is contained in ferritin complexes within the enterocyte and is lost when the enterocyte is replaced.

Question 30

How are water soluble vitamins (vitamins B/C) mainly absorbed?

Answer 30

By Na+ cotransport.

Question 31

Why can vitamins B12 deficiency be caused by gastritis/terminal ileal removal (eg in Crohn’s disease)?

Answer 31

Is absorbed in the terminal ileum bound to intrinsic actor which is secreted by gastric parietal cells.

Question 32

What is coeliac disease?

Answer 32

Intolerance of the gliadin fraction of gluten found in wheat, rye and barley.

Question 33

What consequence does coeliac disease have on the intestines architecture? What does this result in?

Answer 33

Damages the mucosa of the intestines, leading to absence of villi, lengthening of intestinal crypts, and lymphocytes infiltrate the epithelium. These all lead to impaired digestion and malabsorption.

Question 34

What investigations would you do to test a patient for coeliac disease?

Answer 34

Upper GI endoscopy and biopsies of the duodenum to check for mucosal pathology and is villi are reduced or absent.
Blood tests to serology, electrolyte imbalances and anaemia.

Question 35

How is coeliac disease treated?

Answer 35

Diet - cut out gluten.

Question 36

Give 3 symptoms of coeliac disease

Answer 36

Diarrhoea.
Flatulence.
Weight loss.
Sensory loss in hands (malabsorption affects nerves).